Thermal pollution in rivers: Will adding gravel help to cool them down?

Abstract

Thermal pollution in rivers can be caused by dams, logging, municipal wastewater treatment, and other human activities. High water termperatures stress ecosystems, kill fish, and promote disease and parasites, and so dam operators, timber companies, and municipalities are held responsible for thermal loading caused by their operations. These entities are looking for ways to mitigate environmental damage. When Portland General Electric (PGE) was applying for re-licensing of its extensive hydroelectric project on the Clackamas River in Oregon, questions were raised about whether the company's existing plans to improve fish habitat on the lower river by adding gravel into the channel to replace lost sediment would also help to bring maximum summer water temperatures within regulatory limits.

A study co-led by a PNW Research Station scientist provided critical information to PGE--and the 33 interested parties that signed off on its re-licensing agreement--about how river overall temperatures are affected as water flows through naturally occuring gravel bars. The research showed that although water emerging from gravel bars tends to be cooler than the main channel, gravel augmentation alone is unlikely to cool the whole river. It could still provide positive benefits, however, by increasing the number of cool spots for fish to hide during the hottest part of the day.

Based on science by Gordon Grant, and Barbara Burkholder

Citation

Oliver, Marie. 2011. Thermal pollution in rivers: Will adding gravel help to cool them down? Science Findings 133. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 6 p.